DAT18-07: The aversive nature of withdrawal represents a powerful source of negative reinforcement, perpetuating the use and abuse of oxycodone and other prescription opioids. More effective strategies to relieve and prevent withdrawal may decrease consumption of prescription opioids, and facilitate efforts to discontinue use and prevent relapse. These strategies must be informed by a deeper understanding of the neural circuits mediating aversion and other facets of prescription opioid withdrawal. The long-term goal of our research is to determine how opioid exposure and withdrawal modify nucleus accumbens inhibitory microcircuits, and ultimately use this knowledge to reverse or prevent maladaptive changes that contribute to addiction. The nucleus accumbens is commonly associated with reward but also has a ?dark side?, contributing to the aversive aspects of opioid withdrawal and other states of aversion. The specific goals of this proposal are to evaluate the contribution of nucleus accumbens fast-spiking interneurons (FSIs) and medium spiny neurons (MSNs) to aversive behavior during oxycodone withdrawal, and determine how oxycodone withdrawal modifies cellular properties of FSIs and MSNs. The scientific premise for this proposal is based on published and preliminary data that nucleus accumbens FSIs and D2-MSNs are activated during opioid withdrawal and regulate aversive states. Our central hypothesis is that FSIs are inhibited by opioid exposure and exhibit rebound activation during opioid withdrawal, modulating aversion through their GABAergic synapses onto MSNs. We predict that chronic oxycodone exposure reorganizes synaptic output of FSIs onto MSNs, changing how FSIs regulate aversion. In AIM 1, we will determine how FSIs, D2-MSNs, and D1-MSNs regulate aversion during oxycodone withdrawal. Using a clinically relevant model of spontaneous oxycodone withdrawal, we will use chemogenetic methods to manipulate the activity of FSIs and MSNs, and measure conditioned place aversion as well as classic somatic signs of withdrawal. We will also determine how FSI manipulations regulate the activation of MSNs. We predict that FSI activation constrains the expression of oxycodone withdrawal through an inhibitory influence on D2-MSNs. In AIM 2, we will determine how the cellular properties of FSIs and MSNs are altered by oxycodone withdrawal. After continuous oxycodone exposure for one week, we will prepare acute brain slices in the presence of oxycodone, and precipitate withdrawal ex vivo by exposing the brain slice to naloxone. We expect to find an increase of GABA release from FSIs onto D2-MSNs during withdrawal, a neuroplastic change that would explain why FSIs constrain aversion during withdrawal. We will also measure the trajectory of cellular changes after repeated withdrawals in vivo, and predict the cyclical engagement of FSIs and MSNs during each withdrawal episode will generate enduring and maladaptive neuroplasticity in the nucleus accumbens. Successful completion of these experiments will uncover a novel role for nucleus accumbens FSIs in opioid effects, and indicate these cells represent a new therapeutic target for alleviating states of opioid withdrawal.